Does ZrF “complexation” occur in magmas?

Abstract

The local environment around Zr (0.8–3 wt%) was investigated in quenched glasses (albitic, basaltic, dacitic and rhyolitic) using XAFS (X-ray absorption fine structure) spectroscopy at the Zr K-edge. The local environment around Zr in F-free glasses is characterized by two contributions, arising from O nearest neighbors ( ∼ 6 at 2.07–2.08 ± 0.01 Å) and next-nearest Si neighbors at ∼ 3.68 ± 0.05 Å. In the most highly polymerized glass (rhyolitic composition), the significantly larger mean ZrO distance (2.10 ± 0.01 Å) and the appearance of ZrZr contributions at ∼ 3.67 ± 0.05 Å are interpreted as evidence for the appearance of minor amounts of 8-coordinated Zr. In F-bearing albitic glass, no clear evidence was found for F first neighbors around Zr. Nonetheless, F is likely to affect the medium-range environment around Zr in this silicate glass because of the depolymerization induced by this halogen. The local structure around Zr in this set of glasses is very similar to that found in synthetic melts up to 1500 K and in natural glasses from various origins throughout the world. The possibility for REE-F complexes is also discussed.